BackgroundThe Hein ("Swallow"
in Japanese) was the only Japanese Army Air Force (JAAF) fighter with a inverted "V" liquid cooled engine.
The Ki-61 prototype was completed in December 1941, and flew well, reaching
a
speed of 368 mph. During the first half of 1942 the prototype
was extensively tested, performing very well against a captured P-40E
Warhawk and a German
Messerschmidt Bf-109E sent to Japan by submarine. The Tony was the first
Japanese fighter to enter production after the start of the war in the Pacific,
and the only inline engined fighter produced.

Production
A total of 3,078 were built. All Ki-61s were assembled at the Kagamigahara Plant. Most of the Ki-100s were assembled at the Kagamigahara Plant, as well. Twelve Ki-100-Ib aircraft were assembled at the company's Miyakonojo Plant.

First Combat - Dolittle Raid Intercept Interestingly, its
first combat was over Japan when a test model flown by Lt. Umekawa happed
to be flying on April 18, 1942 during the Dolittle Raid. He attacked a B-25
but had to break off due to lack of fuel and problems with his guns. On the
B-25s crew reported the sighting, and this was mistakenly interpreted as
evidence that the Japanese were importing or producing copies of the Messerchmidt
109.

Liquid Cooled Engine Most Japanese Army and Navy aircraft
were equipped with radial engines, while the Tony was one of the first
with
a liquid
cooled
engine. In 1937 Kawasaki had purchased a licence to build the German
Daimler Benz DB.601 engine, used in the Messerschmitt 109. Kawasaki
revised and lightened the engine, creating their own new design in 1940
the Ha-40, inverted V-12 engine. Around this engine Kawasaki planned
the Ki-60 fighter, the production design being a lighter aircraft designated
the Ki-61.

Mistaken For Messerchmidt Bf-109
Initially in combat, the Tony was thought to be a German Messerschmidt Bf-109, due to its similar silhouette and in-line engine. Later, Allied intelligence learned that this type was an entirely new Japan design.

Combat Usage As the A6M
Zero had
done for the Navy, the Ki-61 did for the Army. It could out
run, dive and climb every Allied fighter in 1943 except the P-38
Lightning. It
was the first fighters to be equipped with “fire-proof jacket" for
fuel tanks, (early form of self-sealing fuel tanks inferior to Allied self sealing fuel tanks,
but offered some protection, plus armor plate behind the pilot's head.
It was rugged and able to withstand more damage than other Army fighter.
But, it was plagued with mechanical and engine problems when flying
in the tropics, and also a shortage of supplies and trained ground
crews to maintain it. Many were lost in the air or on the ground
in New Guinea. During home defense, the aircraft were up-gunned and
used to intercept B-29s and for Taiatari (body
crashing) attacks of ramming American bombers.

Mauser 20mm Cannon In Tony The Japanese army imported 800 Mauser 20mm cannon and ammunition for use in 400 Ki-61s. To distinguish the planes equipped with the Mauser cannon, Kawasaki and the army identified these planes by assigning four-digit MNs made by adding the base number of 3000 to each plane's true serial number: 3001-3400. The Japanese did not manufacture any cannon based upon these Mauser cannon. All of the Mauser cannon in Ki-61s were the German-made imports. Apparently no spares were initially on hand, but as planes were written off the cannon were removed and used as spares. Some reports say that 388 instead of 400 Ki-61s carried the Mauser cannon and that some of those were converted from earlier Ki-61s by field modification, but this has not been confirmed.